[performance-profiler] Optimize prefixNumDate digit matching hot path

## Hot Path
`libbeat/common/match/matchers.go:164-209`, specifically `(*prefixNumDate).Match`, which validates each date digit with repeated range comparisons. The CPU profile identified this function at **65.02s flat / 79.14s cumulative (8.98%)**.

## Profiling Data
The same benchmark command was run before and after a temporary, behavior-preserving digit-loop optimization:

```text
go test -run '^$' -bench='BenchmarkPatterns/Name=startsWithDate2,_Matcher=Match' -benchmem -count=5

Before median: 203.3 ns/op
After median: 178.8 ns/op
Both: 0 B/op, 0 allocs/op
```

## Proposed Change
Optimize the digit-validation loop in `(*prefixNumDate).Match` while preserving the current prefix, separator, suffix, and ASCII digit semantics. The temporary implementation used for measurement reduced the per-digit loop overhead without changing the matcher API or behavior.

## Results
**Improvement:** 12.0% lower median latency on the targeted `startsWithDate2` matcher benchmark, with allocations unchanged at zero.

## Verification
- `go test ./...` passed after the temporary optimization.
- The existing matcher benchmark and behavior tests passed.
- The temporary change was removed after measurement; the repository worktree is unchanged.
- Duplicate search found no existing issue or open PR for `prefixNumDate` or this date-matcher optimization.

## Evidence
- Hot path: `libbeat/common/match/matchers.go:164-209`, `(*prefixNumDate).Match`.
- Benchmark harness: `libbeat/common/match/matcher_bench_test.go:98-141`; the `startsWithDate2` pattern is defined at line 108.
- CPU profile command: `go test -bench=. -benchmem -cpuprofile=cpu.prof -memprofile=mem.prof -count=5`.
- Targeted before/after command: `go test -run '^$' -bench='BenchmarkPatterns/Name=startsWithDate2,_Matcher=Match' -benchmem -count=5`.

- [ ] Replace the repeated digit checks in `(*prefixNumDate).Match` with the measured optimized loop.
- [ ] Retain and run matcher behavior tests and the targeted benchmark to confirm the improvement on CI hardware.

---
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Start in libbeat/common/match/matchers.go:164-209 at (*prefixNumDate).Match, then read the startsWithDate2 benchmark in libbeat/common/match/matcher_bench_test.go:98-141. Replace the repeated digit checks while preserving prefix, separator, suffix, and ASCII digit behavior. Run the matcher behavior tests, the targeted benchmark, and go test ./...; done means the tests pass and the benchmark improvement is confirmed on CI hardware.

Written by the indexing model from the issue text.